1. The events

Under the provisions of a treaty signed by Nero, the crown of the subordinate kingdom of Armenia was given to a Parthian prince by Rome.¹ In 113 AD, however, king of Parthia Chosroes I (or Osroes) overthrew his nephew Axidares (or Exedares) and proclaimed Parthamasires, Axidares’ elder brother, king of Armenia.² Chosroes’ action was probably an attempt at avoiding a claim on his throne by Parthamasires, who was the first-born son of the previous Parthian monarch, but he was supplanted at the time of succession by his uncle.³ Chosroes’ violation of the treaty was the spark for the beginning of war.

Trajan left Rome in October 113 AD.⁴ Upon arriving in Athens, he met Chosroes’ envoys, who presented their king’s peace proposal on the condition that Parthamasires would be acknowledged. The emperor, however, gave no clear indication of his intentions. Trajan arrived in Asia Minor, most probably in Ephesus, and visited Seleuceia of Cilicia, before arriving in Antioch in January 114 AD.⁵ In spring, Trajan left for Melitene in eastern Cappadocia, the headquarters of legion XII Fulminata. After an emperor’s decree, Melitene was upgraded to the metropolis of the region.⁶

2. The forces

The legions that took part in the Parthian war were at least 11. Legions II Trajana, III Galica and IV Scythica had already been stationed in Syria. Added to them were III Cyrenaica from Egypt, VI Ferrata from Arabia and X Fretensis from Judaea. Armed forces from the provinces of Danube were also assembled in Cappadocia, the campground of XII Fulminata and XVI Flavia Firma. Among them were I Adiutrix, VII Claudia and XV Apollinaris. An unknown number of detachments from other units, such as I Italica, V Macedonica, XII Gemina and XXX Ulpia, were additionally sent. The number of allied units (auxilia) that took part in the operations is also unknown. In theory, their number should have been equal to that of the legionnaires.⁷ All in all, Trajan’s army must have consisted of at least 110,000 men. There is no evidence about the opponent, Parthian army at first and rebels later on.

3. The campaign

After seizing Arsamosata without a fight, the Roman army was assembled in Satala. Rulers of various nations of the Black Sea and Caucasus arrived there and paid homage to Trajan.⁸ While the invasion force was advancing in Armenia unperturbed, Parthamasires was desperately asking Trajan for a meeting. Their meeting finally took place in Elegeia (modern Ilica). In front of the entire army, the Parthian prince took off his crown and left it at the emperor’s feet, hoping to get it back. This did not happen though. Parthamasires protested that he had not come as a prisoner. He had come voluntarily in order to be crowned king of Armenia. Trajan responded that Armenia belonged to the Romans. He initially allowed Parthamasires to leave under a Roman escort,⁹ but Parthamasires was murdered on the road, probably under orders from Trajan himself.¹⁰

The whole of Armenia soon submitted without any major resistance and was organized into a province.¹¹ The newly-established province included Cappadocia and Lesser Armenia. Former consul L. Catilius Severus was appointed governor.¹² For the aforementioned successes, Trajan was awarded the title of Optimus, most probably in September 114 AD.¹³

At the same time, the emperor turned southwards into Mesopotamia and seized Nisibis (modern Nusayabin) and the strong fortress of Adenystra,¹⁴ whereas in winter, general Lucius Quietus seized Sigara and other regions without a fight, probably reaching as far as Libana.¹⁵ In the winter of 114/5 AD, Trajan stayed in Edessa (modern Urfa), where the ruler of Osroene, Abgar, handed over the region, thus retaining his throne.¹⁶ Until then, the resistance was minimal due to the superiority of the roman force and the internal parthian conflicts. Chosroes was unable to control his vassal rulers. Apart from those who submitted to Trajan without a fight, some demanded an agreement with Trajan, while others had rebelled against their king. Moreover, Chosroes had to face a contender to his throne.¹⁷ Taking advantage of the chaotic situation, the Roman army advanced even further in 115 AD and seized the lands of the Carduchians and the Marcomedes.¹⁸ At the end of the year, Anthemousia, a neighbouring state of Osroene,¹⁹ was also annexed, whereas some operations must have taken place to the south, towards the Euphrates River and Dura-Europos.²⁰ Trajan thus added the province of Mesopotamia to the empire.²¹ In the winter of 115/6 AD, Trajan returned to Antioch and run the risk of dying when a massive earthquake destroyed the city in December.²²

For the aforementioned victories in northern Mesopotamia, Trajan was awarded the title of Parthicus in February 116 AD.²³ In spring, the army transported the boats they had built in Nisibis along the Tigris River and crossed the river with the use of a pontoon bridge despite Meharaspes’, the ruler of Adiabene, resistance. The whole of Adiabene was soon conquered and became the province of Assyria. The Roman army then advanced towards the Parthian capital. Ctesiphon fell at the beginning of the summer of 116 AD.²⁴ In Rome, the enthusiastic senate gave Trajan the right to celebrate as many triumphs as he liked.²⁵ Meanwhile, the emperor advanced to the Persian gulf coast.²⁶ The national trauma following the fall of Ctesiphon and the popular discontent towards the Roman administration brought a temporary restoration of the Parthian unity, whereas the dispersion of roman forces facilitated the general revolt that broke out in the middle of the summer. Its suppression was extremely difficult and not entirely successful. Mesopotamia remained in roman hands, but Assyria was probably lost.²⁷ When the situation was finally under partial control, Trajan created a subordinate Parthian kingdom in the south at the end of 116 AD and crowned a Parthian prince named Parthamaspates, who had earlier defected to the Roman camp.²⁸ The battles continued in Armenia and part of the region was given to prince Vologases in return for peace.²⁹

In the spring of 117 AD, Trajan failed to conquer Atra, which was still occupied by rebels, and started having health problems.³⁰ Moreover, the Judean revolt³¹ required the dispatch of units to new fronts. In July, he left for Rome, leaving Hadrian in Antioch, under orders to continue the war.³² The emperor died in Selinus (Traianopolis) of Cilicia in August.³³ Hadrian, his successor, abandoned Armenia and Mesopotamia and moved the border to the Euphrates River.³⁴

4. The causes

The contemporary view mentions financial motives. According to it, the whole operation aimed at controlling the trade routes of Mesopotamia.³⁵ Of course, financial motives should not be overlooked, since great profits would result from the expansion of trade, taxes and cultivable land, but there are no indications that they were the main and primary reasons of the campaign.³⁶